Electronic circuits often operate synchronously, under the control of one or more clock signals. The speed of operations can be increased by increasing clock frequencies until signal propagation and other delays become long enough in relation to the clock periods that signals are no longer reliably received within the correct cycle. (Operational frequency is also correlated with power consumption, supply voltage and heat generation, so power and thermal effects may also limit the maximum speed at which a circuit can be operated.)
Some complex circuits, such as microprocessors and digital signal processors, exhibit significant device-to-device variability in maximum clock frequency due to variations in manufacturing conditions, material properties, and other factors. Such devices can be graded after manufacturing by testing them under worst-case conditions with increasing clock frequencies. After determining the frequency at which the device begins to operate inconsistently, a safety factor is applied and the device is certified for use at a particular, lower frequency.
This method of setting maximum operational frequency is fairly reliable (given a sufficiently wide guard band) but may give up performance when the device is operated under better-than-worst-case conditions. Furthermore, static, manufacture-time grading fails to account for the natural change over time in the characteristics of the device.